1. Field of Invention
This invention relates to semiconductor process and more particularly to etching silicon dioxide for forming side wall spacers on gate structures.
2. Description of Related Art
The art of etching openings or side walls in an insulator like silicon dioxide can be accomplished with near vertical side walls. This is accomplished through a highly selective process that has a high preference to etching silicon dioxide. The selectivity of the process can be varied by varying the chemistry and pressure of the process. Control of the steepness of the side walls of an opening into the silicon dioxide is less than perfect using the mixture of gases or pressure of the process.
In U.S. Pat. No. 5,021,121 (Groechel et al.) an improved RIE process is disclosed for etching holes in a layer of silicon dioxide with a high contact angle using a highly selective process. A mixture of an inert gas along with CHF.sub.3 and CF.sub.4 is flowed into a RIE chamber and the wafers being processed are subjected to a magnetic field parallel to the surface of the wafers. In U.S. Pat. No. 5,242,538 (Hamrah et al.) a RIE etch process is disclosed including hydrogen radicals. Addition of these hydrogen radicals, such as ammonia or methane, in amounts from about 5 to about 20 percent will increase the oxide etch rate while suppressing the polysilicon etch rate. C. Y. Chang and S. M. Sze in "ULSI Technology" by McGraw-Hill Company, INC., copyright 1997, pp 346-349 discusses oxide etch process using magnetic fields.
Although the selectivity for etching SiO.sub.2 can be controlled by the gas ratio of CF.sub.4 to CHF.sub.3 or the pressure being maintained within the etching chamber, they do not provide a fine vernier by which the width of a side wall spacer can be easily controlled. Also uniformity across a wafer as a result of non-uniform coating of the oxide material cannot be corrected with either the etching gasses or the chamber pressure. The use of a magnetic field parallel to the surface of the wafer being etched produces an etch rate that is higher at the edge of the wafer. Thus compensating for the non-uniformity of the oxide coating of the wafer that is to be etched by enhancing the etch rate at the edge of the wafer compared to the center. Controlling the intensity of the magnetic field permits a fine control of the selectivity for the preference for etching oxide versus silicon. The use of a controlled magnetic field produces well defined side wall spacers of a prescribed width with improved uniformity over the surface of a wafer.